“The Battle for Fort Dunstad” is a Faction Quest in The Elder Scrolls V: Skyrim
Join the men attacking Fort Dunstad
The men will mass down the hill from Fort Dunstad.
They are easily found on the road leading to the Fort itself. The force will attack as soon as the Dragonborn joins them at that location.
Take over Fort Dunstad by defeating the enemy
All of the fighting happens in the exterior of the fort. You will not need to go inside for any part of this quest.
Take your time clearing out each section of the Fort. Remember, you do not need to go inside. Simply fight through the soldiers. Move around with the soldiers you have gone in with.